Chlamydia trachomatis ("CT") is an intracellular bacteria that is the leading cause of preventable infectious blindness (ocular trachoma) in the developing world and of sexually transmitted disease ("STD") in the United States and certain other parts of the developed world. The estimated annual incidence of CT-caused STD is in the millions. While most CT caused disease can be treated with antibiotics, untreated or inadequately treated infections result in hundreds of thousands of cases of pelvic inflammatory disease each year in the United States, alone. Adverse outcomes of pregnancy, ectopic pregnancy and tubal infertility are among the consequences. Moreover, apparent clearance of infection by a given serovar (serologically distinct strain of CT) can be followed by the infection becoming latent and prolonged or by re-infection. This is important because much CT-caused pathology results from tissue-damaging inflammatory responses of the immune system that are triggered by repeated or prolonged exposures to the whole organism. Therefore, there is a need for: (i) means to detect signs of prior or of persistent covert infection in individuals who have pelvic inflammatory disease or its sequelae listed above; and (ii) means to prevent primary and repeat infections.
Thus, means have been sought to test for CT in humans, to monitor the effectiveness of antibiotic treatment, and to detect signs of covert infection. Means have also been sought to manipulate the immune system (e.g. by vaccination) to prevent CT infections.
Some tests to determine the presence of CT already exist. For example, some DNA hybridization probe tests are known. However, these tests are not well suited to detecting evidence of tissue-damaging immune responses once the number of live organisms becomes small, as in many individuals with CT-caused pathologies.
With respect to vaccines, intact killed bacteria have been tried in human volunteers, but without success (pathological side effects/inadequate protection). The art is also aware that antibody responses are directed at the surface-exposed MOMP of CT. Thus, MOMP has been a focus of vaccine-based research for some time.
Sequence analysis of MOMP has revealed that amino acid sequence variation between serovar isolates accounts for the antigenic diversity of this pathogen. See E. Peterson et al., 18 Nuc. Acids. Res. 3414 (1990) (nucleotide sequence of serovar E) and M. Ishizaki et al., 60 Infect. & Immun. 3714-3718 (1992). The disclosure of these publications, and of all other publications referred herein, are incorporated by reference as if fully set forth herein.
Unfortunately, whole MOMP is too difficult to isolate from natural CT cultures in large quantities that are sufficiently pure for use in mass vaccination. Larger quantities of recombinant MOMP could theoretically be produced in E. coli, but the chemical properties (e.g. insolubility except in detergents) impede its large scale preparation as a non-toxic vaccine. In any event, use of whole MOMP has too much risk of adverse side effects.
Attempts have therefore been made to develop vaccines based on MOMP fragments. For example, papers have been published describing use of certain peptide fragments of MOMP to raise antigenic responses to certain serovars (A,B,C) in mice. See e.g. H.Su et al., 172 J. Exp. Med. 203-212 (1990) (serovar A); J. Allen et al., 147 J. Immunol. 674-679 (1991) (serovar B); M.Ishizaki et al., 60 Infect. & Immun. 3714-3718 (1992) (serovars B, C); G.Zhong et al., 151 J. Immunol. 3728-3736 (1993) (serovar B).
However, the value of these studies in mouse in identifying potential vaccine components for humans is now known to be very limited. Only one MOMP epitope to which mouse lymphocytes respond may correspond to one of at least nine epitopes that we have found to activate human T lymphocytes. Also, antigenic MOMP fragments identified in these murine experiments even had limited predictive value among mice. In this regard, fragments recognized by the lymphocytes of one strain were often not recognized by other mouse strains.
A. Stagg et al., 79 Immunol. 1-9 (1993) attempted to locate antigenic fragments in serovar L in human cell experiments. They exposed, in vitro, T-cells from naive individuals to five peptides and looked for proliferation responses. They identified a strong antigenic region "213-224" partially overlapping a variable (serovar specific region of serovar L), as well as several weakly antigenic fragments "116-127", "135-146" and "274-285".
Apart from serovar specificity issues, humans have a variety of MHC class II types, each type determining the specific antigenic groups to which an individual's immune system can respond. Thus, what is antigenic for one human MHC type, may not be antigenic for others. This is a particularly troubling problem for those seeking to develop vaccines for the human population in general.
As such, a need exists for short antigenic MOMP fragments that are present in most or all Chlamydia trachomatis serovars and also are recognized by most human MHC types.